The Siemens method is known as a method of producing polycrystalline silicon used as a feedstock of monocrystalline silicon for a semiconductor or silicon for a solar cell. The Siemens method is a method of bringing a source gas containing chlorosilane into contact with a heated silicon core wire and thereby growing polycrystalline silicon on a surface of the silicon core wire in a vapor phase using a CVD (Chemical Vapor Deposition) method.
When polycrystalline silicon is grown in a vapor phase using the Siemens method, silicon core wires are assembled into a guard frame shape consisting of the two wires in a vertical direction and one wire in a horizontal direction in a reaction space constituted by an upper structure referred to as a bell jar and a lower structure referred to as a base plate (bottom plate), and both ends of the silicon core wires of the guard frame shape are secured on a pair of metal electrodes placed on the base plate through a pair of core wire holders made of carbon. A supply port of a source gas and an exhaust port of a reaction exhaust gas are also placed on the bottom plate. Such a configuration is disclosed in, for example, Japanese Patent Laid-Open No. 2011-68553 (Patent Literature 1).
Generally, several tens of silicon core wires of a guard frame shape that are secured to a pair of metal electrodes placed on a bottom plate are provided in a reactor, and arranged in a multi-ring form. In recent years, it has been desired to obtain a larger amount of polycrystalline silicon in one batch with increasing demand of polycrystalline silicon. This causes the tendency to increase a size of a reactor, and increase the number of silicon core wires arranged in the reactor. These points are disclosed in detail in Japanese Patent Laid-Open No. 2003-128492 (Patent Literature 2).
However, increasing the number of silicon core wires provided in the reactor to increase an amount of production per batch makes it difficult to stably supply a source gas to a surface of the silicon core wire (a surface of a polycrystalline silicon rod). Unstable supply of the source gas easily causes irregularities referred to as “popcorn” in the surface of the silicon rod, and results in differences in diameter (thickness) of the silicon rod of, for example, 1 mm to 5 mm in a length direction and a defect of shape.
Also, a surface area of each irregularity (each kernel) is 20 mm2 to 200 mm2, but a crack-like clearance (so-called void) reaching the inside of the silicon rod may be created between kernels. Polycrystalline silicon is washed before shipment, and a washing agent entering the clearance is hardly removed, thereby significantly reducing washing efficiency. Further, the clearance in the polycrystalline silicon prevents uniform melting in a growing process of silicon monocrystalline.
In order to prevent occurrence of such popcorn, Patent Literature 2 described above proposes a method of increasing a supply amount of a source gas as a surface area of a silicon rod increases with progression of a precipitation reaction to maintain a temperature of a silicon rod surface within a certain range over the entire period of the precipitation reaction, and maintain a constant concentration of a silicon feedstock on the silicon rod surface.
Japanese Patent Laid-Open No. 11-43317 (Patent Literature 3) proposes a method of once significantly reducing a surface temperature of a silicon rod when a crystal grain having a large diameter is easily generated, and controlling a precipitation condition so that only a crystal grain having a small diameter is generated.
There is also a proposal to prevent generation of a crystal grain having a large diameter by gradually reducing a reaction temperature set at the start of a precipitation reaction, although the proposal relates to an extremely early study in the times when a polycrystalline silicon rod was produced using a single silicon core wire (Patent Literature 4: Japanese Patent Laid-Open No. 55-15999).